Context: Neuromuscular deficits are common in people with chronic
ankle instability (CAI). Corticomotor pathways are very influential in
the production of voluntary muscle function, yet these pathways have not
been evaluated in people with CAI.

Objective: To determine if corticomotor excitability of the
fibularis longus (FL) differs between individuals with unilateral CAI
and matched control participants without CAI.

Main Outcome Measure(s): Transcranial magnetic stimulation was
performed over the motor cortex on neurons corresponding with the FL.
All testing was performed with the participant in a seated position with
a slightly flexed knee joint and the ankle secured in 10[degrees] of
plantar flexion. The resting motor threshold (RMT), which was expressed
as a percentage of 2 T, was considered the lowest amount of magnetic
energy that would induce an FL motor evoked potential equal to or
greater than 20 [micro]V, as measured with surface electromyography, on
7 consecutive stimuli. In addition, the Functional Ankle Disability
Index (FADI) and FADI Sport were used to assess self-reported function.

Conclusions: Higher bilateral RMTs may indicate deficits in FL
corticomotor excitability in people with CAI. In addition, a moderate
correlation between RMT and FADI suggests that cortical excitability
deficits may be influential in altering function.

Ankle sprains are the most common injury in physically active
people and account for approximately 23% (1) and 15% (2) of all injuries
in high school and collegiate athletics, respectively. In addition,
628000 ankle sprains are treated annually in emergency facilities within
the United States, representing approximately 20% of all injuries
treated with emergency care. (3) Further evidence indicates two-thirds
of all ankle injuries are left untreated by health care professionals,
suggesting that the true incidence of ankle injury in the United States
is greater than initial estimates. (3) After an initial sprain, some
individuals have sequelae injuries (4) and report a lack of stability
around the ankle that decreases overall function. (5) The rate of
recurrent ankle injury might be as high as 28.3% (6) and might be due to
a condition termed chronic ankle instability (CAI). The knowledge
regarding the CAI phenomenon is evolving, and most often it is described
as a residual clinical problem in which patients exhibit repetitive
lateral ankle instability resulting in multiple ankle sprains. (7,8)
Current theory suggests that CAI comprises factors related to mechanical
instability, perceived instability, and recurrent sprain. Hertel (7)
comprehensively described many factors that likely contribute to CAI.
Specifically, neuromuscular adaptations of the lower extremity after an
acute ankle sprain have been thought to be major factors contributing to
the CAI and its inherent disability. (7)

Neuromuscular alterations, such as diminished reflex excitability
of stabilizing muscles, (9,10) might contribute to the clinical
impairments that affect gait, (11-14) balance, (15,16) and perceived
function (5,17) and may be risk factors for developing osteoarthritis of
the ankle. (18) Whereas the origin of this neuromuscular dysfunction is
not completely understood, researchers have hypothesized that mechanisms
within the central nervous system might modulate motor control in people
with CAI. (9,10,19,20) Unfortunately, only limited information is
available about neural motor alterations in people with CAI. Most
information regarding these neural alterations has been associated with
spinal reflex differences in ankle-stabilizing muscles after acute
lateral ankle sprains (21) and in individuals with CAI. (9) Recently,
researchers (22) have demonstrated neuromuscular alterations exist in
the proximal quadriceps musculature of patients with CAI, which might
indicate the involvement of multiple neural centers. Whereas spinal
reflexive mechanisms have demonstrated differences after ankle injury,
(10) motor alterations arising from the motor cortex have not been
studied in this population. A better understanding of the neurologic
origins of the clinical impairments that patients with CAI have is
imperative to developing successful intervention strategies.

Corticomotor excitability commonly is assessed using transcranial
magnetic stimulation (TMS), which evokes a stimulus over the motor
cortex and allows for evaluation of descending corticospinal motor
pathways in the corresponding musculature. (23,24) Corticomotor
evaluation allows for the evaluation of the excitability of the motor
cortex in the brain and of descending spinal tracts that influence the
production of voluntary human movement. Investigators have found that
the corticomotor excitability of leg muscles is sensitive to changes in
posture control (25) and is involved with anticipatory postural
reactions. (26) Whereas postural control deficits are common in people
with ankle injury, (27) we do not know if corticomotor excitability is
altered in people with ankle instability or if corticomotor excitability
of leg muscles affects self-reported function.

Therefore, the purpose of our study was to determine if
corticomotor excitability of the fibularis longus (FL) differs between
individuals with unilateral CAI and matched control participants without
CAI. To our knowledge, we are the first to investigate corticomotor
excitability in patients with CAI; therefore, we evaluated the FL
because it has been found to exhibit reflexive dysfunction in similar
patients (9) and its function might be important for resisting inversion
of the ankle. We also investigated how corticomotor excitability of the
FL relates to self-reported function in individuals with and without
CAI. We hypothesized that FL excitability would be lower in the injured
ankle than the uninjured ankle in people with CAI and would be lower in
both ankles of people with CAI than in the ankles of healthy matched
control participants. We also hypothesized that decreased corticomotor
excitability would be related to decreased self-reported function.

METHODS

This case-control study involved 2 groups of individuals: those
with unilateral CAI and matched control participants without CAI.
Corticomotor excitability was tested bilaterally in both groups using
resting motor thresholds (RMTs) of the FL muscle, and self-reported
function was assessed with the Functional Ankle Disability Index (FADI)
and FADI Sport. (17) The control group was assigned an
"injured" limb for matching purposes, and the order of leg
tested was randomized in both groups. The same investigator (B.G.P.)
conducted outcome measures in all participants.

Participants

Twenty-three people initially volunteered for this study. Three
potential participants were excluded because bilateral measurable motor
evoked potentials could not be elicited (1 with CAI, 2 without CAI);
therefore, 20 participants were included in the final data analysis.
Demographic data are provided in the Table. Participants had no
neurologic or muscular disease; history of brain or cranial surgeries,
migraines, or concussion in the 6 months before the study; or history of
knee injury or surgery. Participants with CAI had a history of at least
2 unilateral ankle sprains and decreased self-reported function (FADI
< 90%, FADI Sport <80%). (17) No participant with CAI had sprained
his or her ankle in the 6 weeks before testing. In addition, no
participant reported symptoms of instability for the uninjured ankle in
the CAI group or for either ankle in the control group (FADI > 95%,
FADI Sport > 85%). Healthy control participants without CAI were
matched to participants with CAI based on sex, age, height, and mass. To
limit the potential bias of limb dominance (dominant, nondominant), we
matched the injured ankle in the healthy group to that of the CAI
counterpart based on side dominance instead of merely matching by side
(right and left limb). For example, if a participant with CAI had an
injured nondominant leg, the nondominant leg of the corresponding
healthy matched participant would be assigned as the injured limb
regardless of the side (right, left) on which the injury had occurred.
We defined the dominant limb as the limb with which participants
reported they preferred to kick a ball. All participants provided
written informed consent, and the study was approved by the
Institutional Review Board of the University of Toledo.

Experimental Procedures

Participants were positioned on a dynamometer seat (Biodex System
II Pro; Biodex Medical Systems, Shirley, NY) with their hips flexed to
85[degrees], knees flexed to approximately 10[degrees], and testing
ankles plantar flexed to 10[degrees]. Although force was not being
measured, the dynamometer provided a means of maintaining constant
patient positioning. The calcaneus was secured in a rubber heel cup
mounted on a flat platform (Figure). The superolateral leg inferior to
the knee joint was shaved, debrided, and cleaned with alcohol before
application of the electromyography (EMG) electrodes. Disposable, 10-mm,
pregelled Ag/AgCl electrodes (BIOPAC Systems, Inc, Goleta, CA) were
applied, and the signal was amplified with a gain of 1000 (EMG 100C;
BIOPAC Systems, Inc) before being converted digitally with a 16-bit
data-acquisition system (MP150; BIOPAC Systems, Inc). The EMG signal was
collected at 2 kHz with a common-mode rejection ratio of 110 dB, a noise
voltage of 0.2 [micro]V, and an input impedance of 1 M[OMEGA]). Two EMG
electrodes were adhered 1.75 cm apart on the greatest bulk of the FL,
approximately 2 to 3 cm inferior to the fibular head (Figure). (28,29)
The reference electrode was positioned on the medial malleolus of the
nondominant leg. AcqKnowledge software (version 3.7.3; BIOPAC Systems,
Inc) was used to visualize the EMG and force signal.

[FIGURE OMITTED]

Transcranial Magnetic Stimulation

Before testing, participants donned a Lycra swim cap (Sprint
Aquatics; Rothhammer International Inc, San Luis Obispo, CA) and
earplugs (Aearo Co, Indianapolis, IN) to muffle the sound of the TMS.
Two landmark lines were drawn on the swim cap: l sagittally separating
hemispheres and 1 intersecting the sagittal line coronally at the
external auditory meatus. (30) The MagStim (model 200; MagStim Company,
Ltd, Wales, United Kingdom) was used to deliver a single magnetic pulse
with a possible strength of 2 T; the double-cone coil configuration
allowed for a maximum of only 70% of the stimulation (1.4 T). A
double-cone coil was moved anterior to posterior over the vertex of the
skull while the investigator applied a magnetic stimulus of a constant
intensity until the largest peak-to-peak motor evoked potential in the
contralateral FL was found. This point was denoted on the swim cap with
a felt-tipped marker and used as the point for stimulation during RMT
testing. (31) The double-cone coil was secured to the dynamometer with
an articulating arm (196AB-2; Manfrotto Lino, Manfrotto, Italy).

During testing, participants were instructed to focus on an X
marked on the wall in front of the dynamometer. For both legs, FL RMTs
were found by using a protocol previously recommended for lower
extremity corticomotor excitability testing. (32) The magnetic
stimulation was decreased by 5% until no motor evoked potential could be
elicited. Next, the percentage of magnetic stimulation was increased by
1% until 7 consecutive stimuli (32) produced a measureable motor evoked
potential (> 20 [micro]V). (33) The RMT was expressed as a percentage
of 2 T.

Statistical Analyses

Separate independent t tests were performed to determine if
differences in demographics (age, height, mass, FADI, FADI Sport)
existed between groups. Paired-samples t tests were used to evaluate
differences in FADI and FADI Sport between the limbs of the same
participants in the CAI group. A 2 x 2 repeated-measures analysis of
variance was used to determine if differences existed in RMT between
groups (CAI, control) and ankles (injured, uninjured). In addition, we
used separate 1-tailed Pearson product moment correlations to determine
the relationships between RMT and FADI and between RMT and FADI Sport
for the injured ankle of the CAI group and the matched ankle of the
control group. We classified correlation coefficients of 0 to 0.4 as
weak, 0.41 to 0.7 as moderate, and 0.71 to 1.0 as strong. (34) The
[alpha] level was set a priori at .05. All statistical analyses were
performed using SPSS (version 17.0 for Windows; IBM Corporation, Somers,
NY).

RESULTS

We found no differences between groups for age ([t.sub.18] <
0.001, P > .99), mass ([t.sub.18] = 0.67, P = .50), and height
([t.sub.18] = 0.76, P = .46). The FADI ([t.sub.18] = -8.1, P < .001)
and FADI Sport ([t.sub.18] = 3.99, P = .001) scores for the injured
ankles were lower in the CAI group than in the control group. The FADI
([t.sub.18] = -1.1, P < .001) and FADI Sport ([t.sub.18] = 9.39, P
< .001) scores for the uninjured ankles were lower in the CAI group
than in the control group (Table). The FADI ([t.sub.9] = -6.83, P <
.001) and FADI Sport ([t.sub.9] = -9.88, P < .001) scores were lower
for the injured limb than for the uninjured limb in the CAI group but
not in the control group (Table). No between-legs differences were found
for FADI and FADI Sport scores in the control group; inferential
statistics could not be performed for this comparison because means and
measures of variability were identical between legs (Table).

To our knowledge, we are the first to evaluate corticomotor
excitability in the FL of the ankles in individuals with CAI. Our
findings are important because alterations in descending corticospinal
excitability may affect clinical decision making for specific
interventions. Whereas little research is available on RMT in the FL, we
demonstrated RMTs similar to those in previous experiments in which
investigators used comparable methods in the lower extremity musculature
of the quadriceps. (32) Our findings, which demonstrated altered
descending corticospinal pathways in CAI, may suggest the need to
incorporate interventions that target descending corticospinal pathways,
such as TMS (35,36) and biofeedback, (37) and might better address
altered neural pathways originating at the motor cortex.

Increased bilateral RMTs in the FL muscle of those with CAI
indicate decreased descending corticomotor excitability of
ankle-stabilizing muscles. Transcranial magnetic stimulation is a
relatively noninvasive method that can be used to evaluate corticomotor
excitability of different muscles. Magnetic energy penetrates the skull
and excites areas in the motor cortex, triggering a descending neural
response that travels to a corresponding muscle and causes a measurable
contraction. (23) These increased RMTs suggest that a greater exogenous
magnetic stimulus is needed to excite cortical neurons that correspond
with muscles within the periphery. The assumption is that if these
cortical neurons require increased TMS to be excited, patients with CAI
may encounter more difficulty generating motor commands to the FL
muscle. We do not know how an alteration in FL corticomotor excitability
will affect function. In a recent systematic review, Hiller et al (38)
demonstrated that selected neuromuscular impairments and functional
responses, such as ankle muscle strength and muscular response to a
perturbation, are not grossly different between people with and without
CAI. However, more sophisticated tasks incorporating postural control
and gait have shown that people with CAI display deficits, suggesting
that more complicated tasks may be more affected by altered RMT. (38)

The FL may be vital for ankle stabilization because it
eccentrically controls ankle inversion and may play some role in
preventing ankle sprains. Of interest, we observed an altered RMT of the
FL bilaterally in the participants with CAI, yet they reported only
unilateral symptoms of CAI. Whereas uninjured FADI scores were lower in
the CAI group than the control group (Table), means for the uninjured
FADI (96.9%) and FADI Sport (94.6%) in the CAI group were much higher
than the inclusion criteria. Therefore, the uninjured ankle of the CAI
group could be considered functionally asymptomatic but still
demonstrated decreased corticomotor excitability. Investigators have
reported bilateral deficits in movement patterns (21,30) and force
production (39) about the knee in participants with CAI, but those
researchers only quantified self-reported functional deficits in the
injured ankles of the participants and theorized that bilateral
alterations to reorganization in the central nervous system may help to
explain the findings. However, they could not examine specific motor
pathways as we have in this study. Unfortunately, as was the case for
those other researchers, the retrospective study design that we used did
not allow us to determine if the decreased bilateral corticomotor
excitability occurs after unilateral CAI or if the decreased
excitability is a predisposing factor that may lead to chronic ankle
sprains. Researchers should strive to determine if this phenomenon is
present in individuals who eventually have an initial acute ankle sprain
or if this relationship develops after initial ankle sprain and is an
important contributor to CAI and subsequent, repeated ankle conditions.
In addition, further analysis is needed to determine the effect of CAI
on corticomotor excitability of other stabilizing muscles, such as the
anterior tibialis and soleus.

We also evaluated the relationship between the self-reported
function and FL corticomotor excitability in the injured ankles of
participants in both the CAI and control groups. Moderate negative
correlations were found between corticomotor excitability of the FL and
self-reported function. Further examination of the correlations
suggested that RMT of the FL explains 16% ([r.sup.2] = 0.16) of the
variance in the FADI and 19% ([r.sup.2] = 0.19) of the variance in the
FADI Sport. Whereas 16% and 19% may seem small, the FL is only one
muscle responsible for ankle stabilization. Corticomotor dysfunction
possibly exists in other muscles surrounding the ankle, which may share
responsibility in self-reported ankle instability. In addition, this 16%
specifically relates to neuromuscular dysfunction generated by
corticomotor mechanisms, which may not necessarily reflect other spinal
reflex influences reported to be different in patients with CAI. (9,10)

Deficits in spinal reflex excitability have been reported in the
musculature surrounding the ankle in those with CAI and have been
attributed to arthrogenic muscle inhibition. (9) This spinal reflexive
muscle dysfunction may alter neuromuscular control that is vital for
maintenance of postural control (15,16,27) and gait, (11,12) which are
affected in people with CAI. We can speculate that the inability to
voluntarily and reflexively excite stabilizing muscles around the ankle
may contribute to neuromuscular dysfunction that perpetuates CAI in this
population. Researchers have suggested that spinal reflexive deficits
(40) and corticomotor alterations (32,41) occur in the quadriceps after
knee injury or effusion, indicating that a possible combination of
corticomotor and spinal reflexive deficits may contribute to
neuromuscular dysfunction after lower extremity joint injuries. Further
support has been noted in studies in which the authors have reported an
altered motor pattern in the knees of individuals with CAI before
landing, (19,20,42) suggesting a potentially different feed-forward
pattern may be associated with this condition. Feed-forward motor
control relies on the initiation of preemptive strategies that create
anticipatory movements. This differs from feedback strategies that rely
on real-time sensory information to make moment-to-moment alterations in
neuromuscular activation. (43) Wikstrom et al (13) showed that both
feed-forward and feedback neural mechanisms play a role in the gross
neuromuscular alterations in patients with CAI, yet further research is
needed to determine which of these strategies would best be manipulated
to improve function in patients with CAI.

Understanding the effect of joint injuries on specific neural
pathways may be vital in the development of therapeutic interventions
that can target neural mechanisms causing neuromuscular dysfunction.
Many recent therapeutic advances have targeted spinal reflex inhibition
(44-46) in the quadriceps after knee joint injury, yet modalities that
specifically influence corticomotor pathways are less advanced at this
time. (35,36) In addition, few researchers have assessed the effects of
therapeutic interventions targeting muscle inhibition around the ankle.
(47) Our finding that corticomotor excitability of the FL is diminished
in those with CAI may support the need for clinicians to consider
targeting the motor cortex with effective interventions. Transcranial
magnetic stimulation has been used to enhance neuromuscular function in
the quadriceps after knee injury, (35,36) but the efficacy of this
intervention has not been assessed in muscles surrounding the ankle. An
alternate approach may be the use of biofeedback during strength
training, (48) which has been used to enhance muscle strength by
attempting to involve increased cortical control.

Our study had limitations. Whereas RMT is a common and central (33)
outcome measure to assess corticomotor excitability, a plethora of
outcome measures can be evaluated via TMS. Livingston and Ingersoll (31)
assessed the magnitude and latency of motor evoked potentials and have
evaluated the physiologic reaction of the motor evoked potential
amplitudes in reaction to increasing the magnitude of the stimulus.
Using a battery of outcome measures in future studies may provide more
unique information that can increase our understanding about
corticomotor excitability after joint injury. Furthermore, this data
collection occurred during rest in a somewhat static condition. With
future testing, researchers may assess corticomotor excitability during
an active muscle contraction or during movement, which may reveal more
about cortical muscle control during functional activity. Whereas
corticomotor excitability of the FL does explain 16% of the variance
associated with self-reported function, the self-reported function of
other muscles, such as the tibialis anterior and surae, should be
evaluated to understand the collective contribution of corticomotor
excitability in various lower extremity muscles that stabilize the
ankle. Researchers also may assess the relationship between cortical and
spinal reflex control of muscles during movement after joint injury to
determine the most influential neural pathways for therapeutic
intervention.

The number of magnetic stimuli evoked in each participant varied
depending on the ease of locating the optimal stimulating position and
the specific nature of determining individual RMTs. Whereas we used
previously published methods to minimize the amount of stimuli to locate
the RMT in a systematic process, (32) the repeated TMS possibly affected
the thresholds of cortical neurons. Researchers have used TMS to alter
neuromuscular outcomes, (35,36) but many of them used higher magnetic
outputs and active muscle contractions. Therefore, the potential effect
that the testing measure may have had on evaluating RMT remains unknown,
yet the current methods seem to be underpowered to produce a therapeutic
effect.

CONCLUSIONS

We provided a unique assessment of corticomotor pathways that may
be involved in altering neuromuscular function of stabilizing muscles in
people with CAI. Specifically, our data indicated increased bilateral FL
RMT in patients with CAI. The presence of increased RMT may be of
specific interest to clinicians because this measure seems to have a
moderate relationship with self-reported function.